Injection lasers have been proposed as a radiation source which can be used to transmit information by modulating the laser driving current with data signals representing the intelligence to be transmitted. Such lasers have a nonlinear operating characteristic wherein optical output increases slowly with increases in driving current up to a threshold, above which threshold, optical output increases more rapidly with respect to driving current. To satisfy the joint requirements of maximizing the signal to noise ratio and the on/off ratio, it has been proposed to operate the laser at the "knee" of the operating characteristic.
Thus, the laser driving current is composed of a DC bias current and a data signal current, and desirably the DC bias current operates the laser at the "knee" of the characteristic.
In prior art laser control systems, for example as disclosed in "Bell System Technical Journal", Vol. 57, No. 6, (July-August, 1978), it is known to control laser output by irradiating a photodiode with a pulsed laser and applying the corresponding current output of the photodiode to an RC filter circuit that generates a delayed signal corresponding to the average power of the laser. The average power signal is then compared to a reference power signal and a corresponding error signal is generated and is applied to control the DC bias current of the laser.
Such prior art feedback control circuits have the disadvantage that the DC bias current is generated in response to an average power measurement and, therefore, the bias current is influenced by the data pattern being transmitted.
Thus, prior art laser control systems do not operate efficiently when the laser is held in a particular operational state for a period of time and is then switched to the opposite operational state. For example, if a laser is maintained in an on state, the RC filter circuit of a prior art control system will generate an average power signal and a corresponding DC bias current that is too high for the laser's continuous state of operation, since the prior art circuit cannot distinguish a laser that is continuously on and a laser that is intermittently turned on at a relatively high level. Furthermore, when the laser is turned off, the DC bias current will be maintained at an excessively high level for a period of time corresponding to the RC time constant of the filter circuit.
Alternatively, if the laser is maintained in an off state for a period of time, and, thereafter, the laser is turned on again, the prior art control circuit will not initially provide a sufficiently high DC bias current since the average power signal of the circuit will not reflect the instantaneous value of the optical output of the laser.
Prior art systems also have the disadvantage that a relatively high back bias voltage, for example, 10 to 20 volts, is required for a photodiode in order to ensure that a sufficiently distinctive waveform is provided at the output of the photodiode in response to the radiation of the laser.
Accordingly, it is a primary object of the invention to provide a feedback circuit for controlling the peak optical output power rather than the average optical output power of an injection laser.
A further object of the invention is to provide a feedback control circuit that operates to generate a DC bias feedback current that closely follows the operational "on" condition of a laser despite changes in the duty cycle or pattern of the operational on/off data signals for the laser.
Another object of the invention is to provide a feedback control circuit for controlling the peak optical output of a laser wherein a photodiode is operated with a relatively small reverse bias voltage.
A further object of the invention is to provide a feedback control circuit that holds the peak optical output of a laser constant when a loss of data occurs and that maintains a constant output until a data input is resumed.
These and other objects of this invention will become apparent from a review of the detailed specification which follows and a consideration of the accompanying drawings.